One class of narrow-aisle lift trucks employs a pair of unpowered non-steerable front wheels, or load wheels, a steerable powered drive wheel assembly rigidly mounted near one rear corner of the truck, and an unpowered vertically-sprung idler wheel assembly near the other rear corner of the truck. With all four wheels mounted on the same base frame, one wheel must be vertically sprung, or floor irregularities could result in loss of traction by the drive wheel. In some applications the vertically-sprung idler wheel assembly uses a free-wheeling, non-steered caster wheel which is self-steering. One early form of truck of that type is shown in U.S. Pat. No. 2,564,002. In various other applications the sprung idler wheel is not castered, but instead steered via a linkage. A truck of this latter type is shown in U.S. Pat. No. 3,392,797.
The suspended wheel is suspended from the frame of the truck by coil springs, a torsion bar or leaf springs as shown and described in U.S. Pat. No. 4,813,512, which is hereby incorporated by reference for its description of such devices. Lift trucks achieve significant economies when vehicle frames of a uniform type are used with either a castered idler wheel or a linkage-steered idler wheel. Provision of an idler wheel mounting arrangement which will readily accommodate either type of steering is disclosed in U.S. Pat. No. 3,392,797. In the idler wheel mounting arrangements disclosed in that patent, the pivot steering axis of the idler wheel is located somewhat inwardly from a lateral extremity of the truck to allow space for a castered wheel to swing. The springs used to oppose weight on the idler wheel must be aligned with the pivot or steering axis, so that they do not impose moments which would cause undue bearing wear, and hence the springs also must be located undesirably inwardly from the lateral extremity of the truck, where they tend to interfere with provisions of an unobstructed operator compartment and waste space.
One problem with prior art lift trucks is that they sway when the truck stops abruptly or abruptly changes direction or both. While such motion will not tip the truck, it can be disconcerting to an operator. Normally an operator will slow down and allow the tilt to naturally dissipate before resuming travel. Accordingly, such unwanted tilting or swaying reduces the efficiency of the operator and the overall productivity of lift truck operations.
U.S. Pat. No. 5,685,555 describes one method for providing a suspended idler wheel mounting arrangement wherein the suspension means has its motion dampened in order to limit the tilt of a lift truck following an abrupt stop or an abrupt change in direction. Here, a mechanical inertial damper is coupled between the suspended wheel and the frame. The inertial damper includes a pair of parallel outer plates, with a slider plate disposed between the plates. A pair of friction pads is provided between an outer plate and the slider plate, and frictionally engages the slider plate when the frame moves relative to the wheel to slow the relative motion between the frame and the wheel. An adjustable means, such as a belville washer or spring, is provided for adjusting pressure of the outer plates on the slider plate.
While this prior art system is effective in providing stability to the vehicle, this system can provide only a single level of damping during use, and thus cannot dynamically adjust for variations that occur in the height of the mast or the weight of the load. The present invention addresses these issues.